1. Field of the Invention
The present invention relates to an antilocking vehicle braking system and method for stopping vehicles without wheel lock. The present invention has been found to be particularly useful in the automobile braking art, especially as a device for individual tire skid control, and hence, will be discussed with particular reference thereto. However, the present invention is applicable to many other types of braking system as well, not only automobiles and trucks, as long as fluid braking systems are used in combination with wheels, and the term "automobile" is used herein to include vehicles of all sorts such as airplanes, tractors, and trucks.
2. Description of the Prior Art
When a brake is applied to the wheel of a vehicle, there is a tendency for the surface velocity of the tire to be reduced to below the velocity of the vehicle. As the ratio of these velocities is reduced, i.e., as the tire begins to slip on the road, braking force begins to increase to a maximum when the tire surface velocity is from about 10% to about 30% below vehicle velocity, but beyond about 30% slip, braking force decreases substantially, until the wheel is locked from rotating. Thus, it is apparent that as the operator of the vehicle increases the fluid pressure in the braking system, and, thus, the force of the brake shoes on the brake drum, beyond a certain optimum, actual braking force at the tire surface begins to decrease, thereby putting the vehicle in danger of a skid.
To reduce the likelihood of such a skid, antilock braking systems have been developed. Antilocking braking systems usually include means to control the flow of fluid between the brake fluid supply and the individual brakes based on a sensing of a skid or wheel lock condition. This involves the transmitting of an indication of vehicle motion under braking conditions from the tire to cause generation of a control signal. This transmission and the operation of a control system by the signal must be done in a manner such that environmental factors will not affect the signal transmitted, the device must be reliable in operation for a long period of time and consistent in its indication, and vehicle operation must not be impeded and made unsafe through failure of the system.
Several types of antilocking vehicle braking systems and methods have been known and used before, and typical examples thereof in the automobile brake system art are shown in U.S. Pat. 3,735,844, issued May 29, 1973, H. N. Barnes; U.S. Pat. 3,813,130, issued May 28, 1974, to M. Inada; U.S. Pat. 2,257,628, issued Sept. 30, 1941, to N. E. Wahlberg; U.S. Pat. 2,471,956, issued May 31, 1949, to J. W. F. Holl; U.S. Pat. 3,011,831, issued Dec. 5, 1961, to H. W. Trevaskis; U.S. Pat. 3,611,284, issued Oct. 5, 1971, to J. P. G. Lewis; U.S. Pat. 3,529,872, issued Sept. 22, 1970, to R. E. Mitton; and U.S. Pat. 3,542,164, issued Nov. 24, 1970, to F. R. Mortimer.
The Inada, Wahlberg, Holl, Mitton and Trevaskis antiskid brake devices utilize multiple wheel control which does not permit individual adaptation of torque per wheel and wheel brake for conditions encountered by that wheel.
In addition, the Inada device uses an electrical control system, as does the Mitton device, which is not desirable under some circumstances. Also, several of the systems use motion sensors in the individual wheels of the vehicle to determine torque imbalance.
The Trevaskis device also vents brake fluid from the brake supply without shutting off the brake supply in a manner that causes a sudden change in braking force, which is operably undesirable.
The Lewis device, although operating with each individual wheel, uses wheel motion to sense braking problems through electrical means. It also uses on/off control to regulate power from the central brake fluid supply without linearly lowering the braking force which result in sudden changes in braking force, which is operably undesirable.
The Barnes device reacts individually to each wheel. It, however, although totally hydraulic, does not gradually lower braking pressure to the desired amount but has volumes for lowering its pressure thereby producing an uneven and intermittent braking effect.